A fuel cell is a device that converts energy of a chemical reaction into electrical energy (electrochemical device) without combustion. A fuel cell generally comprises an anode, cathode, electrolyte, backing layers, and current collectors. Since the voltage of a typical fuel cell is usually small, they are often stacked in series. In such configurations, fuel cells can have 2-3 times greater efficiency than internal combustion engines.
There are several types of fuel cells, which are typically classified by their various electrolytes. One common type of fuel cell is a Proton Exchange Membrane (PEM) fuel cell. PEM fuel cells generally involve a solid organic polymer (e.g., polyperfluoro-sulfonic acid or NAFION®) as an electrolyte. They have high power density and can vary output quickly, which makes them desirable for portable and auto applications. PEM fuel cells are also known as polymer electrolyte fuel cells, polymer electrolyte membrane fuel cells (PEMFC), solid polymer electrolyte (SPE) fuel cells, and solid polymer membrane (SPM) fuel cells.
Fuel cells produce electricity, water, and heat using fuel and oxygen. The oxidation and reduction reactions occurring within a fuel cell are:2H2→4H++4e−1 oxidation half reaction4e−1+4H++O2→2H2O reduction half reactionThis electrochemical process is a non-combustion process that does not generate airborne pollutants. Water (liquid and vapor) is the only emission when hydrogen is the fuel. Therefore, fuel cells are a clean, low emission, and highly efficient source of energy that can use abundant and/or renewable fuels.
The two half-reactions normally proceed very slowly at the low operating temperature of a fuel cell. Specifically, kinetic performance of PEM fuel cells is limited primarily by the slow rate of the O2 reduction half reaction (cathode reaction), which is more than 100 times slower than the H2 oxidation half reaction (anode reaction). The O2 reduction half reaction is also limited by mass transfer issues. Thus, catalysts are typically used on one or both the anode and cathode to increase the rates of each half reaction. Platinum (Pt) has been the most effective noble metal catalyst to date because it is able to generate high enough rates of O2 reduction at the relatively low temperatures of the PEM fuel cells.
The catalysts used to induce the desired electrochemical reactions are often incorporated at the electrode/electrolyte interface by coating a slurry of the catalyst particles onto the electrolyte surface. When hydrogen or methanol fuel feed through the anode catalyst/electrolyte interface, an electrochemical reaction occurs, generating electrons and protons (hydrogen ions). The electrons, which cannot pass through the polymer electrolyte membrane, flow from the anode to the cathode through an external circuit containing a motor or other electrical load, which consumes the power generated by the cell. The protons generated at the anode catalyst migrate through the polymer electrolyte membrane to the cathode. At the cathode catalyst interface, the protons combine with electrons and oxygen to give water.
One major challenge for fuel cell development and commercialization has been the supply of fuel to the fuel cell. While hydrogen gas is generally the most efficient fuel, the use of hydrogen gas is complicated by storage concerns. For example, in order to supply significant amounts of hydrogen gas, especially for portable fuel cells, the hydrogen gas must be stored under pressure in specialized tanks. Such pressurized containers can add weight and complexity to a fuel cell apparatus, in addition to the costs associated with purifying and compressing hydrogen gas. Another concern regarding hydrogen gas is that it can easily ignite.
A Direct Methanol Fuel Cell (DMFC) is a popular type of PEM fuel cell that uses methanol for fuel. DMFC's are the only commercially available fuel cell units today. While DMFC's solve the hydrogen storage dilemma and perform well in the field, DMFC's suffer from lower cell voltages than are available with hydrogen gas fuel, and possess inherent toxicity and flammability difficulties. Also, the use of methanol (and fossil fuels in general) as fuel fails to eliminate carbon dioxide release, and they produce small levels of by-products that can poison the fuel cell and degrade performance. Furthermore, methanol fuels usually contain H2SO4 to facilitate oxidation of methanol and to provide ionic conductivity in the catalyst. The H2SO4 penetrates the anode structure providing ionic conductivity throughout the electrode, thus allowing most of the catalyst to be utilized resulting in improved performance. The use of H2SO4 is, however, undesirable due to sulfate species adsorbing onto the electrode surface and also the corrosive nature of the acid. Moreover, significant work has been undertaken by others to develop reformers to convert a variety of fossil fuels and other alcohols to hydrogen, but the weight burden and complexity of this approach is very large and has generally been rejected for automotive and small fuel cell applications.
In another approach, hydrogen fuel is stored in the form of metal hydrides, which release hydrogen gas to the fuel cell upon hydrolysis of the metal hydride. While the storage of hydrogen in metal hydrides overcomes the carbon dioxide issue, the maximum storage efficiency obtained thus far is about 4.0 wt. %. Other disadvantages of these systems are the necessity to carry water and, most importantly, the requisite use of expensive metal hydrides. Further, the metal hydrides are irreversibly hydrolyzed into metal hydroxides during hydrogen production. Thus, these systems require handling of metal hydroxide by-products, which are difficult, energy intensive, and costly to convert back to the original metal hydride form.
The United States Department of Energy (DOE) has identified hydrogen storage energy density as a critical requirement for the successful transition to the hydrogen economy. And to encourage efforts to overcome the challenges associated with hydrogen fuel, the DOE has established the hydrogen storage efficiency targets identified below.
Year200520102015Specific Energy kWh/kg1.52.03.0kg H2/kg System4.56.09.0Energy Density kWh/l1.21.52.7Gm H2/l System364581Storage System Cost $/kWh642$/kg H2 capacity20013367Refueling Rate kgH2/min0.51.52.0Loss of usable H2 (g/hr)/kg stored10.10.05
The use of hydrogen as a sole fuel and as a supplementary fuel has been adopted by several engine manufacturers including Ford and BMW. The use of hydrogen can allow very lean combustion, which dramatically improves fuel economy and reduces harmful exhaust emissions. As a supplementary fuel, the use of hydrogen in amounts from about 1 to about 10% can facilitate the use of higher amounts of exhaust gas recirculation to reduce harmful emissions without incurring combustion instability. Hydrogen is also being pursued as an effective agent to enable the adoption of Lean NOx catalyst. The hydrogen can initiate catalytic reactions at lower temperatures, and has no carbon burden.
In light of the current difficulties with hydrogen generation and storage and the increasing need for a clean source of energy, new hydrogen generation and storage technologies for portable and stationary fuels cells are needed. Specifically desired are technologies that offer low pressure, high density storage of hydrogen and which overcome the efficiency, performance, and toxicity concerns of methanol, metal hydrides, and other hydrogen sources for fuel cells. The compositions, methods, and devices disclosed herein address these and other needs.